PLC with two control loops for ramping up and ramping down, how?

sevun · Aug 12, 2016

I've got a control loop controlling the speed of a pump with a VFD as a function of a flow meter measured GPM. When the GPM is increasing, I want the response of the pump speed to gradually speed up (PI loop with big I). When the GPM is decreasing, I want the pump to immediately slow down (PI loop with little I).

Right now all the keywords I'm searching for are bring up unrelated topics. Suggestions for where to look or what the magic keywords to search for is appreciated.

Peter Nachtwey · Aug 12, 2016

What not ramp the flow set point up using a ramp function and ramp the flow set point down faster? You shouldn't need to change the gains.

I can tell you how to change the gains to lengthen or shorten the ramp times but the ramps will be exponential not linear. In other word the exponential ramps will change faster at the start and slower near the end of the ramp.

NetNathan · Aug 12, 2016

Use digital bit?
GPM Increasing (DI = 1)
GPM decreasing (DI = 0)
Use DI to reconfigure PID (Change Integral term)...but this may upset the PID by changing Integral on the fly.

Do you have the PID resolved? Is it working okay using either a Large Integral or small Integral?

NetNathan · Aug 12, 2016

Peter Nachtwey said:
What not ramp the flow set point up using a ramp function and ramp the flow set point down faster? You shouldn't need to change the gains.

I can tell you how to change the gains to lengthen or shorten the ramp times but the ramps will be exponential not linear. In other word the exponential ramps will change faster at the start and slower near the end of the ramp.

Ramp would be better choice...then you are not writing into PID on the fly.

sevun · Aug 12, 2016

So the set point variable can have one ramp (slow rise) when the process variable is increasing and another ramp (fast fall) when the process variable is decreasing?

sevun · Aug 12, 2016

NetNathan said:
Use digital bit?
GPM Increasing (DI = 1)
GPM decreasing (DI = 0)
Use DI to reconfigure PID (Change Integral term)...but this may upset the PID by changing Integral on the fly.

Do you have the PID resolved? Is it working okay using either a Large Integral or small Integral?

No, I'm heading out next week to implement the system. I've seen the concepts and played with PID controllers some, but this will be the first time implementing one.

sevun · Aug 12, 2016

SCORE! I'm using Schneider-Electric PLCs and there is a RAMP control block which has two separate variable options for inc_rate and dec_rate. I'm guessing I feed the set point through that!! That's for everyone's help.

rupej · Aug 13, 2016

The super easy way would be to just use the VFD's built in ramp function, setting ramp up to 10 seconds or so, and ramp down to 1 second.

PLC with two control loops for ramping up and ramping down, how?

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